Change wspoon PGOOD polling interval to 500ms

Increase the frequency of checking for PGOOD drops
to the help the fault analysis code analyze faults closer
to the time of failure and also enable the system to
do a clean power off faster, where it's desired to stop
applying power as soon as possible to faulted components.

Change-Id: I6d44aa841e7aca6731c3c82d78c5275b5f225617
Signed-off-by: Matt Spinler <spinler@us.ibm.com>